Magnetic recording technique has been widely utilized in various fields such as video equipment, audio equipment and computers because of its excellent advantages which cannot be seen in other recording systems. That is, it enables repeated use of media, it can easily use a signal in an electronic form, enabling the configuration of a system combined with peripheral, and it can modify a signal.
In order to cope with the tendency toward miniaturization of equipment, the demand for improvement in the quality of recorded and reproduced signals, the demand for prolongation of recordable time and the demand for increase in the recording capacity, it has been desired to further improve the recording density, reliability and durability of recording media.
In audio and video applications, for example, in order to cope with the development of a digital recording system realizing improved sound quality and picture quality into practical use and the development of a video recording system adapted for high definition TV, magnetic recording media enabling the recording and reproduction of short wave signals and having an excellent reliability and durability against the increase in the speed of the head relative to the medium have been required more than required in the conventional systems.
In order to allow such a coating type magnetic recording medium to perform high density recording, various methods have been studied and proposed to use an iron powder or a magnetic alloy powder mainly comprising iron instead of magnetic iron oxide powder which has heretofore been used, improve the magnetic properties of the magnetic layer by improving the fineness, packing and orientability of magnetic powder, enhance the dispersibility of ferromagnetic powder, or improve the surface properties of the magnetic layer.
For example, a method which comprises using a ferromagnetic metal powder or hexagonal ferrite powder as a ferromagnetic powder to improve the magnetic properties thereof is disclosed in JP-A-58-122623 (The term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-61-74137, JP-B-62-49656 (The term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-60-50323, U.S. Pat. Nos. 4,629,653, 4,666,770, and 4,543,198.
In order to enhance the dispersibility of a ferromagnetic powder, the use of various surface active agents is proposed in JP-A-52-156606, JP-A-53-15803, and JP-A-53-116114. Further, the use of various reactive coupling agents is proposed in JP-A-49-59608, JP-A-56-58135, and JP-B-62-28489.
Further, in order to improve the surface properties of a magnetic layer, the improvement in the method for forming and treating the surface of a magnetic layer after the coating and drying of the magnetic layer is proposed in JP-B-60-44725.
In order to secure output in a short recording wavelength range, efforts have been made to enhance the coercive force of the magnetic recording medium or increase the packing of the magnetic recording medium to provide a higher magnetic flux density. In another approach, the thickness of the magnetic layer is reduced to reduce the self-demagnetization thereof and hence secure output in a short recording wavelength range.
For example, JP-A-63-187418 discloses a wet-on-wet process magnetic recording medium composed of an lower nonmagnetic layer and an upper magnetic layer, the upper magnetic layer being made of iron oxide and having a thickness of 0.5 .mu.m at least.
In recent years, digital recording of video signal has been studied. In VTR system for consumer use such as DVC (digital video for consumer use), the miniaturization of hard ware involving the cost reduction and easy operation is essential. To this end, the use of a band compression technique has been proposed for hard ware part. For the part of magnetic recording medium, the reduction of the width and thickness of video tape and the width of recording track has been proposed.
As a result, the tape width has been reduced to 1/4 in., which is half that of the current VHS video tape. The tape thickness has been reduced-to 7 .mu.m. The recording track width has been reduced to 10 .mu.m (corresponding to that of the long time recording mode of 8 mm VTR tape). In the proposed magnetic recording medium, the recording current and other specifications are determined on the basis of metal evaporation tape. The proposed magnetic recording medium is further characterized by the ability to simultaneously recording a signal having a wavelength as short as 0.5 .mu.m and a servo signal having a wavelength of 45 times that of the former and the track width which is narrower than that of the magnetic head.
In the foregoing recording system, signals are partially overwritten by means of a magnetic head having a width greater than that of recording track to record in a narrow track a signal containing a component having a wavelength of about 50 times the shortest recording wavelength. In this recording system, it is necessary that a new short wavelength signal be recorded (overwritten) on the magnetic layer while a long wavelength signal such as servo signal recorded deep in the magnetic layer being erased by a short wavelength recording signal. Heretofore, a magnetic layer having an enhanced coercive force has been used to increase the short wavelength output. However, in the case where a digital video signal containing a long wavelength component is recorded, the coercive force of the magnetic layer cannot be raised without limitation from the standpoint of the foregoing overwritability.
In other words, a problem has arisen that the distribution of coercive force in the magnetic layer must be controlled so that written signals can be erased when overwriting is effected at a high output in a long wavelength range even in a short wavelength recording for high density recording.
JP-A-5-109061 discloses that a ferromagnetic metal powder is preferably used as a magnetic material for the purpose of improving the overwritability in digital recording on a magnetic disk having a magnetic layer thickness of not more than 0.5 .mu.m and a coercive force of not less than 1,400 Oe (oersted). JP-A-5-120676 discloses a proposal similar to that described above.
JP-A-5-242462, JP-A-5-274651, and JP-A-5-290359 disclose that a magnetic recording medium having excellent high band characteristics, overwritability, corrosion resistance and light transmission in the form of 8 mm video tape consisting of an upper magnetic layer and a lower nonmagnetic or high permeability layer can be provided.
However, JP-A-63-187418 discloses the reduction of a magnetic layer to not more than 2 .mu.m but does not refer to the foregoing requirement for overwritability. JP-A-5-109061 and JP-A-5-120676 do not refer to controlling the distribution of coercive force in the magnetic layer to improve the overwritability.
JP-A-5-242462, JP-A-5-274651, and JP-A-5-290359 disclose improvements in overwritability. However, the short wavelength signal and long wavelength signal which are overwritable are actually such that the ratio of the former to the latter is small as 4.5, e.g., 2 MHz and 9 MHz. Further, there is no reference to the foregoing problem. Thus, the requirement that a signal having a wavelength as short as 0.5 .mu.m can be secured at a high output while a servo signal having a wavelength of 45 times that of the former is being recorded cannot be met.